1. Field of the Invention
The present invention relates to a process for treating bauxite, which is widely used as a raw material for producing alumina. More particularly, the present invention relates to a process for treating bauxite in an environmentally friendly manner, which process converts an insoluble residue (called xe2x80x9cred mudxe2x80x9d)xe2x80x94which is produced when bauxite is treated by means of Bayer""s processxe2x80x94into a composition which is effectively employed, and enables recycling of an alkali component in Bayer""s process, the alkali component being contained in the insoluble residue and inhibiting the effective use of the residue.
2. Description of the Related Art
Conventionally, an insoluble bauxite residue is produced in very large amounts in Bayer""s process. In Bayer""s process, bauxite is treated with an alkali aluminate solution at high temperature and high pressure, a bauxite residue is separated from the resultant solution and removed, aluminum hydroxide is precipitated in and separated from the aluminate solution containing an alkali component in large amounts, and the aluminum hydroxide is calcined, to thereby produce alumina. The amount of a bauxite residue which is produced in the process varies with the quality and the treatment process of bauxite. About 600 kg to 1,000 kg of a bauxite residue is produced for every 1 ton of alumina. A bauxite residue contains large amounts of iron oxide, silicon oxide, and aluminum oxide, and thus extensive studies have been performed in an attempt to use the residue as a material such as a raw material for producing iron or a material for construction, and to enable effective use of these components after extraction and separation.
For example, there have been proposed a process for using such a bauxite residue as aggregate and a process for using the residue as a raw material for producing cement. However, alkali contained in a bauxite residue reacts with cement and the cement may become brittle. Therefore, the amount of the residue which is used for cement is limited, and the residue has never been actually used. Incidentally, there has been proposed a process for converting a silicate compound contained in a bauxite residue into a zeolite, and using the zeolite as an adsorbent. However, such a silicate compound contains alkali, and thus after the zeolite which is produced from the compound is used, the thus-used zeolite must be disposed of. Meanwhile, additional waste which must be treated is produced during the conversion of the compound into the zeolite.
In addition to the aforementioned processes, in an attempt to enable effective use of a bauxite residue, extensive, long-term studies have been carried out. However, in many cases, a large amount of a residue after the treatment of bauxite is not effectively used and is disposed of, for the reasons described below. The size of particles of the residue is as small as 4 microns, and thus carrying out dehydration treatment and handling the particles are very difficult. In addition, the bauxite residue contains an alkali component in large amounts.
U.S. Pat. No. 4,045,537 discloses a process for removing alkali contained in a bauxite residue, which is the so-called xe2x80x9csoda lime process.xe2x80x9d In the process, a bauxite residue is mixed with a calcium compound, the mixture is sintered in a reducing atmosphere, and the sintered product is washed and alkali is recovered, to thereby use the product as a raw material for producing iron. However, the treated product contains Na2O in an amount of more than 1 wt % and the product is unsatisfactory as a raw material for producing iron, although large-scale equipment is necessary in the process since all of an insoluble residue is treated. Meanwhile, Japanese Patent Application Laid-Open (kokai) Nos. 6-172876 and 7-166252 disclose a process for subjecting a bauxite residue to mineral dressing or chemical treatment by use of an acid, to thereby produce a raw material composition for iron production. In the process, the amount of alkali contained in the residue is reduced, and the residue can be used as a raw material for iron production. However, the process is unsatisfactory in consideration that a solution after acid treatment and a desilication product containing alkali must be treated.
Substantially, bauxite does not contain Na2O, but red mud usually contains Na2O in an amount of 5 wt % or more, which amount varies with the form and the treatment process of bauxite. Such an alkali component is incorporated into red mud, for the reasons described below. An SiO2 component (which is called reactive silica) of clay predominantly comprising kaolinite, which is contained in bauxite, reacts with alkali in a solution, producing an insoluble desilication product (sodium aluminosilicate) predominantly comprising sodalite. Therefore, a bauxite residue is an insoluble residue which predominantly comprises iron oxide and a desilication product containing an alkali component.
A bauxite residue is produced in large amounts, and thus the residue is used as a material for construction which is consumed in large amounts, such as a raw material of cement, aggregate, or subgrade, or a raw material for iron production. When the residue is used as a raw material of cement, iron oxide, silicon oxide, and aluminum oxide are added to a calcareous raw material, to thereby produce portland cement. In this case, the residue is standardized to contain Na2O in an amount of 1 wt % or less. When the residue is used as a raw material for iron production, the amount of Na2O in the residue must be 1 wt % or less, since problems in operation, such as deterioration of a refractory substance in a shaft furnace, may occur.
However, there has not yet been considered a process for producing a bauxite residue containing as little alkali as possible, and treating bauxite with an alkali aluminate solution without production of additional waste during the production of the residue.
In view that it will not always be possible to dispose of a residue which is produced after the treatment of bauxite, and that disposal of an insoluble residue containing an alkali component may become an environmental issue in the future, an object of the present invention is to develop a new process for treating bauxite, which process comprises removing an alkali component from a bauxite residue without production of additional waste, recovering the alkali component and recycling the same in Bayer""s process, and effectively using the bauxite residue as material, which process is environmentally friendly and industrially effective. The present invention has been accomplished on the basis of the object.
In order to attain the above-described object, the present inventors have performed extensive studies, and have found that, in Bayer""s process in which bauxite is treated with an alkali aluminate solution to thereby produce alumina, when a desilication product and an insoluble residue are separately precipitated without incorporation of the desilication product into a bauxite residue or production of a complex of the desilication product, the insoluble residue other than the desilication product becomes a bauxite residue containing no alkali component, and when the desilication product is treated with a calcium compound and an alkali component is solubilized to thereby separate the alkali component from the desilication product and recover the component, the alkali component can be recycled in Bayer""s process almost completely. The present inventors have also found that the bauxite residue contains an alkali component in very small amounts, and thus the residue can be effectively used as a resource. The present invention has been accomplished on the basis of these findings.
Accordingly, the present invention provides the following process.
(1) A process for treating bauxite characterized by comprising:
a) a step for extracting an alumina component from bauxite with an alkali aqueous solution under conditions for suppressing the production of a desilication product to the utmost, and separating the resultant solution into a first insoluble residue containing alkali in small amounts and an aqueous solution containing alumina and silica components;
b) a step for adding seeds to the thus-separated aqueous solution to thereby precipitate a desilication product, and separating the solution into the solid desilication product and an aqueous solution containing an alumina component; and
c) a step for mixing the thus-separated desilication product and a calcium compound, calcining the mixture, and washing the calcined product with water and filtering the resultant aqueous solution, to thereby separate the solution into a second insoluble residue containing an alkali in reduced amounts and an aqueous solution containing an alkali component.
(2) A process for treating bauxite according to (1), wherein the aqueous solution in step c) contains an alkali component together with an alumina component.
(3) A process for treating bauxite according to (1) or (2), wherein the step for extracting the alumina component from bauxite is carried out under conditions in which the alumina component is dissolved as much as possible and the dissolution of a silica component is suppressed as much as possible.
(4) A process for treating bauxite according to (1) or (2), wherein the step for extracting the alumina component from bauxite is carried out under conditions in which the dissolution of the alumina component is suppressed as much as possible and a silica component is preferentially eluted.
(5) A process for treating bauxite according to (4), which further comprises a step for treating the first insoluble residue with an alkali aqueous solution again, for extraction of an alumina component.
(6) A process for treating bauxite according to any of (1) through (5), wherein precipitation of a desilication product in the first insoluble residue is prevented by carrying out solid-liquid separation immediately after the alumina component is extracted from bauxite with the alkali aqueous solution under conditions for suppressing the production of a desilication product to the utmost.
(7) A process for treating bauxite according to any of (1) through (6), wherein the alkali is sodium hydroxide.
(8) A process for treating bauxite according to any of (1) through (7), wherein the first insoluble residue contains alkali in an amount of about 1 wt % or less.
(9) A process for treating bauxite according to any one of (1) through (8), wherein the first insoluble residue is used as a raw material for iron production.
(10) A process for treating bauxite according to any one of (1) through (9), wherein the calcium compound includes at least one species selected from among limestone, quicklime, slaked lime, and calcium carbonate.
(11) A process for treating bauxite according to any one of (1) through (10), wherein, when the calcium compound which is added to a desilication product containing a silica component is reduced to CaO, the mol ratio of CaO to SiO2 (CaO/SiO2) is about 2-5, and the calcining is carried out at about 1,150-1,350xc2x0 C.
(12) A process for treating bauxite according to any one of (1) through (11), wherein the desilication product and the calcium compound which are calcined have a particle size of about 60 xcexcm or less.
(13) A process for treating bauxite according to any one of (1) through (12), wherein the desilication product and the calcium compound are granulized in advance, for suppressing dust generation during said calcination.
(14) a process for treating bauxite according to any one of (1) through (13), wherein heat generated during said calcination is used in a process for producing aluminum hydroxide or alumina from bauxite.
(15) A process for treating bauxite according to any one of (1) through (14), wherein an organic impurity which is generated and accumulated in a process for producing aluminum hydroxide or alumina from bauxite is incinerated during said calcination.
(16) A process for treating bauxite according to any one of (1) through (15), wherein the second insoluble residue contains alkali in an amount of about 1 wt % or less.
(17) A process for treating bauxite according to any one of (1) through (16), wherein the second insoluble residue is used as a raw material of cement.
(18) A process for treating bauxite according to any one of (1) through (17), wherein the desilication product is sodalite or a substance predominantly containing sodalite.
(19) A process for treating bauxite according to any one of (1) through (18), wherein the alkali component which is recovered in step c) is recycled in the process for treating bauxite.